Knights of the Round Table (1953)

Other mistake: When King Arthur confronts Mordred in the Abbey ruins on the cliff tops, you can see two fighter jet planes flying up the coastline behind him.

Lancelot: Aye, it is the valley of death... the Devil himself has plowed it under.

Trivia: Anne Crawford plays Morgan Le Fey, the mother of Modred (Stanley Baker). In real life, Crawford was only eight years older than Baker.